Summary The hematopoietic system is particularly sensitive to stimuli that cause DNA replication stress, which can result in DNA breaks. Defects in the ability to properly respond to this stress are linked to a predisposition to hematologic malignancies, hematopoietic cell deficiencies, and bone marrow failure. Recent advances include the identification of genes that respond to DNA replication stress and a greater understanding of genes necessary for hematopoiesis of specific lineages, but much remains unknown about both processes and how they intersect to cause pathology. Moreover, DNA replication stress is observed in human cancers, is induced by oncogenes such as Myc, causes DNA damage, and contributes to genome instability, but it remains poorly understood. Smarcal1 and more recently, Zranb3 were identified and evaluated biochemically and by in vitro studies to localize to replication forks and be critical for responding to DNA replication stress. Smarcal1 and Zranb3 are fork remodelers and reverse replication forks to allow repair of damaged DNA and rewind single-stranded DNA. Both Smarcal1 and Zranb3 have similar functions, but biochemical differences are reported. However, the functions of Zranb3 and Smarcal1 and whether they are redundant or complementary at times of replication stress or have unique functions in vivo has not been explored. Recently, we genetically demonstrated that Smarcal1 was essential for mediating the effects of acute DNA replication stress on hematopoietic stem and progenitor cells and contributed to the development of T cell lymphoma. Our preliminary data support the hypothesis that Zranb3 contributes to hematopoiesis and lymphomagenesis in ways distinct from Smarcal1, and that Zranb3 and Smarcal1 together contribute to hematopoiesis and lymphoma development. Therefore, we propose three Aims with in vivo genetic approaches to test this hypothesis. 1) Investigate the role of Zranb3 in Myc-induced replication stress and its contribution to acute and chronic replication stress-induced lymphoma development. 2) Determine the role of Zranb3 in hematopoiesis and hematopoietic stem and progenitor cell replication stress. 3) Evaluate the combined loss of function of Zranb3 and Smarcal1 in hematopoiesis and lymphomagenesis. Innovative concepts are tested with a new mouse model and multiple approaches, including high-resolution genome wide end-sequencing. Completion of these Aims will significantly advance understanding into DNA replication stress proteins and their contribution to genome stability, hematopoiesis, and hematological malignancies.